Wind power plants—also referred to as windmills—are a promising source of renewable energy, which is already available on the market. Multiple windmills are often clustered in wind parks or wind farms located in windy areas. The classic design comprises a tower with a horizontally revolvable nacelle at the top, where a rotor with propeller-blades is arranged and where also an electrical generator is located.
In windmill applications, gearboxes are required to translate the rotor-blade rotation at an operational speed of approximately 6 to 20 revolutions per minute (RPM) to the electrical generator which runs in a range of approximately 900 to 2000 RPM. As spur gears in general allow a gear ratio of up to about 1:5 per stage, multiple gear stages are required to achieve the required ratio of gearing. To reduce the number of stages and gearwheels and also because of efficiency, size, noise and cost considerations, wind power stages can alternatively be equipped with at least one planetary gear stage having a higher gear rate. Also combinations of planetary and spur stages are used.
Wind power stations often operate in the Megawatt range and are designed for a lifecycle of about 20 years or more. Although the efficiency of such a gearbox is quite high (e.g. about 98%), in view of the Megawatts of power transmitted, the gearbox itself or the oil inside of it has to be actively cooled in many cases. The long lifetime also means hundreds of millions of rotor revolutions and billions of revolutions of the generator. Analogous applications are also gearboxes in other slow running power stations with slowly rotating input shafts which require a gearing up, like waterwheels, tidal power plants, etc. having similar operating conditions than the above mentioned windmills. In addition to the quite long lifetime, the load of such a windmill can vary in a wide range and rapidly, in particular in case of an emergency stop or a flurry, squall or gust of wind. A load control can be achieved by varying the angle of attack of the blades or by a horizontal rotation of the whole nacelle, but those adjustments are slow compared to the possible changes in wind speed. Also, the environmental and climatic conditions in the nacelle are not favourable for running machinery. The above facts show that a windmill-gearbox will require servicing during runtime.
There are quite sophisticated methods for determining the expectable runtime of a gearbox during the design phase. Those calculations are based on the expected loads and on statistical data and experiences. Due to the uncertainties of the actual load profiles during runtime, manufacturing tolerances and often harsh environmental conditions, those calculations can only be used for determining a preventive maintenances schedule comprising a considerable safety margin. Certain unexpected impacts during runtime, such as peak loads, temperature cycles, manufacturing inaccuracy etc. can cause an early failure which—if not detected in advance—can lead to unexpected gearbox failure like jamming or spinning and all the side effects which might result therefrom.
In windmills, the gearbox is usually placed in the nacelle, high above ground level, and therefore it is a difficult task to maintain or repair the gearbox or parts of it. It is burdensome to access it, in particular as windmills are often sited in remote areas, on mountains or even offshore. In many cases, a helicopter is required to supply the nacelle with spare parts.
In particular, as maintenance and replacement of gearboxes can be quite cost-intense, it is a desire to schedule repairs and/or replacements based on a determined actual condition of the gearbox rather than precautionary do those tasks, often well in advance of the end of the actual lifespan.
A known method to estimate the condition of a gearbox is to analyze the oil inside the gearbox for abrasion, in particular for grazed metal particles. Beside cyclical manual analysis, there are also in-line sensors known. For example, the device presented in KR 2007024230 will provide condition signal and a warning, if the result of the oil-analysis—as indication for worn gearwheels—is critical.
Another method—which also has been done manually by skilled craftsman for ages—is to “listen” to the sound emitted by the gearbox. In automated systems, this can be done by means of acoustic analysis by a kind of microphone or by means of vibrations or acceleration sensors. The publications US 2008/0234964, CN 101 196 174 or U.S. Pat. No. 5,661,659 refer to wear detection systems for mechanical systems like gearboxes by a structural sound or vibration analysis. The publication US 2007/0118333 discloses an abnormality diagnosis system, wherein the acoustic sensors for detecting an abnormality in the gearbox sounds are comprised in a bearing unit.
In WO 2004/034010 a method for quality surveillance in manufacturing gearboxes is presented. This is achieved by monitoring two interacting cogwheels by two synchronously sampled rotary transducers attached to the gearwheel shafts after assembly of the gearbox. Therein, the backlash of the gearing is determined by measurements in a forward and backward mode which gives an indication of the quality of the gearbox.
In U.S. Pat. No. 6,175,793 a vehicle's steering-wheel-gearbox is monitored by a similar angular position sensing means at the input and output shafts of the gearbox and also by a torque sensing unit. By a mathematical cancellation of the torque related torsions, a signal representative of the wear of the gearwheels in the gearbox is generated.
JP 58 034333 relates to a invariably monitoring of stress conditions at a diaphragm coupling on the basis of the extent of axial displacement, the rotational speed and load of a power plant. This is done by a means for detecting the extent of axial displacement near the fitting part of the diaphragm coupling to a generator shaft. On the basis of the displacement, the rotational speed and load of a power plant, stresses at the diaphragm coupling are calculated and safe operation state of the coupling is judged.
Although some wear effects can be concluded from the rotational position data, the wear information comprised in the rotational data is incomplete and the evaluation of those data is based on experience. The play or backlash of the tooth of the gearwheels can quite accurately be estimated, but many wear effects in a gearbox can not be concluded from rotational position offsets only. If only angular measurements are analyzed, wear which primarily manifests itself in other effects, not visible in the angular position, is undetectable.
A more reliable basis for determining the condition of a gearbox is therefore desirable. The above mentioned noise measurement can cover a wider range of wear effects, but it is in general not highly accurate and also comprises guesswork and a setup-specific configuration which is different for each instance of gearbox. Also, the desired management of the whole system, comprising an accurate load monitoring and taking into account dependencies from the actual load conditions and a lifespan- or lifetime-management of critical system parts is not feasible by acoustic analysis only. In particular, in big and difficult to access machineries like the ones in windmill towers, false detections are a big cost issue, no matter if false positive or false negative detections are made. In many critical applications, acoustic analysis on its own is not reliable and accurate enough for a stand-alone use.